The present invention relates to a method and apparatus for electromagnetic detection, particularly to such method and apparatus for use in the measurement of at least one parameter of a fibrous web during manufacture.
The invention has particular application in the manufacture of fibrous webs, such as paper or non-woven sheet material, for use in the medical and hygiene industries. In particular, the invention may be employed for on-line measurement of a parameter, e.g. the moisture content and/or the basis weight, of a fibrous web for providing a control output, e.g. for process or quality control, during web manufacture.
The invention at least in its preferred form described below employs infrared measurement techniques and detectors in the manufacture of fibrous web.
In the specification, the term “parameter” is used to denote the property (moisture content, thickness, basis weight etc.) being measured, the term “sample” is used to denote the portion of the object (substrate, sheet, web etc.) presented to the measurement gauge for measurement, and the term “fibrous web” covers all forms of sheet material comprising fibres compacted together, including paper and non-wovens.
A variety of measurement gauges have previously been employed in the paper and non-woven industries, in processes for continuously manufacturing fibrous web, in order to detect various parameters of the web. For example, it is extremely important to know the weight per unit area of the fibrous web as it is produced, since the density or basis weight of the web may be closely correlated with its ultimate tensile strength, burst strength and water transmittance. It is also extremely important in these industries to monitor the moisture content of the web as it is treated, in order to ensure drying efficiency and the sterility of the ultimate product.
However, the existing techniques in these industries for measuring basis weight and moisture content, including those employing infrared absorption spectroscopy, have hitherto proved unsatisfactory. The interaction of light with the small fibres and voids in the fibrous web is very complex, and this can cause serious measurement issues, particularly in the case of lightweight webs typically having a weight in the range 8-20 gsm and a fibre size of 10-20 μm. For example, light striking the web in the region of the voids will tend to pass through the web with little or no interaction, so that the measurement detector picks up only a faint absorption signature from the target web. And, in the region of the fibres, the light striking the web interacts strongly with the fibres and is scattered, and may often be diverted away from the measurement detector altogether.
Further, the inclusion amongst the measurement signals of signals obtained from light passing straight through the voids and signals obtained from light scattered away from the measurement detector significantly distorts the measurements obtained, and in some cases may result in the generation of a wholly inaccurate spectral absorption characteristic pattern. Such issues are particularly pronounced in the case of lightweight materials where the proportion of voids to fibrous mass is much greater.
Hitherto, no satisfactory measurement gauge has been found to measure parameters of lightweight fibrous web material.
In other industries, infrared absorption gauges are well known and are used for measuring various constituents or parameters of samples or substrates, such as the moisture content of the sample, the thickness or coating weight of a film on a base layer or substrate, or the thickness or basis weight of the sample.
Infrared absorption gauges conventionally operate by projecting infrared radiation at two or more wavelengths onto a sample and measuring the intensity of the radiation reflected, transmitted or scattered by the sample. Signals proportional to the measured intensity are processed to provide a value of the parameter being measured. At least one of the two or more wavelengths projected by the gauge is chosen to be absorbed by the parameter of interest, while at least one other wavelength is chosen to be substantially unaffected by the parameter of interest. For example, when measuring the amount of water in a sample, one of the wavelengths (the “measuring wavelength”) can be chosen at an absorption wavelength of water (either 1.45 micrometer (microns) or 1.94 micrometer (microns)) and the other wavelength (known as the “reference wavelength”) is chosen to be one that is not significantly absorbed by water.
Generally, gauges include an infrared radiation source having a predetermined emission spectrum, and a detector for receiving radiation reflected, transmitted or scattered by the sample; filters are placed between the source and the sample to expose the sample only to the desired measuring and reference wavelengths; in this case, the sample is successively exposed to radiation at the selected wavelengths, e.g. by placing appropriate filters on a rotating wheel in front of the radiation source. Alternatively, a filter wheel can be placed between the sample and the detector, and each filter be successively interposed between the sample and the detector.
The detector measures the intensity of light after interaction with the sample and produces a signal according to the intensity of the radiation incident upon it. In the most simple case, by calculating the ratio between the signal from the detector when receiving light at the measuring wavelength and the signal from the detector when receiving light at the reference wavelength, a measurement signal can be obtained that provides a measure of the parameter concerned, for example the amount of moisture in the sample. Often, several measuring wavelengths and/or several reference wavelengths are used, and the signals of the measuring wavelengths and of the reference wavelengths are used to calculate the parameter concerned.
Such infrared absorption gauges have been proposed for use in the manufacture of fibrous web, but only partially successfully and only in the case of medium and heavyweight materials.
Tests on a lightweight fibrous web, using a standard measurement gauge employing light in the near infrared (NIR) range of 1-2.5 μm and scatter based measurement, found that the measurements obtained were overly sensitive to fillers, fibre size and fibre distribution to produce a robust measurement. At the same time, tests with a standard direct transmission measurement gauge employing light in the near infrared (NIR) and mid infrared (MIR) ranges of 1-4 μm found that the measurement data was very variable and unreliable as a result of the effects of fibre size and web voids.
Thus, significant measurement problems exist in the use of standard infrared measurement gauges in the manufacture of fibrous web, both in the case of direct transmission light and scattered light measurement gauges. Further tests also revealed that measurement problems exist with most fibrous web materials but are particularly pronounced when the web material is a lightweight one.
Accordingly, there is a significant need in the industries for manufacturing fibrous web, such as paper and non-woven sheet material, for a measurement gauge capable of producing accurate measurements of various parameters, irrespective of the weight range of the web and irrespective of the distribution of voids and fibres throughout the web, and especially at the lightweight end of the possible weight ranges where the proportion of voids to fibre mass is high and the fibre density is low.